Several properties of diamond, such as its hardness and thermal conductivity, make it highly desirable for use as a coating or thin-film applied to articles whose life is limited by excessive wear, such as cutting tools. However, because diamond is a brittle material, in the form of a monolith it does not have the toughness of other traditional cutting tool materials, such as tungsten carbide or PDC (polycrystalline diamond compact). Toughness is especially important to the performance of cutting tool materials in environments where impulsive or high impact forces may be involved, for example in interrupted cutting. The use of diamond as a thin-film, or coating, takes advantage of the wear resistant properties of the thin-film while also taking advantage of the bulk properties (toughness) of an underlying substrate base material. However, in order to successfully realize these advantages, the adhesion strength of the diamond film to the underlying substrate must allow the thin-film and substrate to operate as a "composite" system. This may be particularly challenging for some base materials due to thermal expansion mismatch between the film and substrate which gives rise to very large residual stresses. In addition, the chemical composition of some base materials can impair or prevent the formation of strong bonds between the film and substrate. Ignoring these effects can lead to very weak bonding and may result in delamination of the film or coating from the base material of the substrate during use.
One of the most important base materials for various kinds of flat and rotary cutting tools is cemented carbide, such as tungsten carbide (WC) ceramic particles sintered in a matrix of cobalt (Co) binder. The utility of this class of materials is based upon the combination of a hard, abrasive phase (WC grains) which is cemented or bonded by a metal, ductile phase (Co binder). While the metal binder phase gives the cemented carbide toughness, it is this constituent which is primarily responsible for the difficulties encountered in establishing adhesion to CVD diamond films. Under typical conditions of CVD diamond synthesis, the binder phase of cemented carbides, which is commonly cobalt, but may also be iron or nickel, may interact with the gaseous CVD diamond growth species and catalyze the formation of graphitic material instead of or in addition to diamond. The formation of a graphitic layer on the substrate results in poor adhesion between the film and substrate. In addition, during the chemical vapor deposition of diamond films, the binder phase may dissolve the diamond substrate interface, thereby reducing the interfacial contact area between the film and substrate to degrade mechanical bonding. Finally, the mismatch in thermal expansion between the diamond film and substrate typically results in large residual stresses in the diamond film following deposition which further challenges the interface integrity.
Early efforts to improve the adhesion of diamond films to WC--Co materials led researchers to remove cobalt from the surface of WC--Co materials using several techniques. In U.S. Pat. No. 4,731,296, Kikuchi et al. discuss the formation of an "etch layer" with reduced cobalt concentration extending to between 0.1 to 1.0 micrometers (micrometers) into a WC--Co based substrate with 1-4 wt % (weight percent) Co. This method encourages the nucleation and growth of diamond films without the preferential deposition of graphite. However, methods based on the chemical removal of the binder phase have several drawbacks which can influence the utility of the diamond coated article. Removal of the binder phase to a depth which is greater than the general size dimension of the free surface grains results in the formation of an embrittled layer at the surface of the WC--Co article. In the presence of an applied stress, such as the residual stresses imposed on the diamond film following deposition or those encountered during use of the article, failure of the interface by loss of WC grain cohesion or by crack extension in this embrittled area results in delamination. On the other hand, removal of the binder phase to a depth which is less than the general size dimension of the free surface WC grains usually results in interaction between the diamond and binder phase unless a physical barrier to diffusion across the interface is created. Furthermore, these approaches do not have a means of producing a mechanically tough, interfacial crack deflection mechanism which is necessary to provide the interfacial fracture toughness required for the abrasive applications of metal cutting.
Other researchers have recognized that a physical barrier or so-called "diffusion barrier" to diamond/binder interaction may improve adhesion by preventing interaction between the binder phase and the diamond film. Proper selection of such a layer may also reduce residual stresses between the diamond film and the underlying substrate by selection of an interlayer material having a coefficient of thermal expansion with a value between those of the film and underlying substrate. However, the interlayer approach is not preferred because it is complicated, expensive, and does not result in the increase in interfacial toughness which other techniques achieve.
The U.S. Pat. No. 5,415,674 issued to Feistritzer et al. discloses a technique to vaporize and re-crystallize surface WC grains. This process is a significant improvement over methods which produce a sub-surface binder-depleted region. However, this process is carried out at a temperature too low for rapid grain growth of the free surface WC grains. There is no discussion of the important details of free surface chemical composition or structural features of the free surface of the WC--Co which are necessary for adhesion of the diamond film under abrasive conditions as described above.
The U.S. Pat. No. 5,100,703 issued to Saijo discloses a process for treating WC--Co having a binder phase of 4 wt % (weight percent) or less by using a decarburizing gas comprised of oxygen and hydrogen between a temperature of 500 and 1200.degree. C. (centigrade). While decarburization of the free surface WC grains promotes re-carburization during CVD diamond deposition and thus promotes chemical bonding between the diamond film and substrate, the method disclosed in this patent results in a free surface in which the WC grains are smaller than the WC grains in the bulk. This process therefore does not contain the crack deflection or interfacial toughening mechanism essential for highly abrasive applications.
The U.S. Pat. No. 5,648,119 issued to Grab et al. discloses the formation of a roughened substrate to improve the "mechanical component of adhesion". The roughness is provided by subjecting the article to a second, resintering process which causes grain growth of the WC particles which are located at the surface. However, the time and temperature required for adequate grain growth at the surface as described therein also results in some grain growth in the bulk material in addition to a significant reduction in the binder phase composition. Several mechanical properties of this class of materials are proportional to the binder phase content. Therefore, this reduction of the binder content resulting from execution of this process can lead to a reduction in important properties such as the fracture toughness and transverse rupture strength.
There is a need for a process for coating a cemented carbide article with a strongly adherent diamond film by which both the mechanical and chemical components of bonding are optimized without degradation of the substrate bulk properties.